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The father of the Briton accused of trying to assassinate Donald Trump believes his son could have been "blackmailed or put up to it".

Michael Steven Sandford, 20, was arrested on Sunday after he allegedly tried to grab a police officer's gun while at a Las Vegas rally for the presumptive GOP presidential candidate.

According to official papers filed on Tuesday, Sandford "claimed he had been planning to attempt to kill Trump for about a year but decided to act on this occasion because he finally felt confident about trying it". He faces up to 10 years in prison.

Sandford's father, Paul Davey, 50, told the Portsmouth, England-based News that it was an "absolute shock" that his "polite and peaceful boy" has been accused of attempted assassination. He said his son had zero interest in politics and would be at a loss to name the president of the United States.

"Whether he's been blackmailed or put up to it, that's the only thing me and his mum can think of," Davey told the paper. "It's so against his nature, and obviously with his Asperger's, we think somebody has got hold of him and done something."

But Davey also said that the family told US authorities they were concerned about their son.

"He's been refusing to come back, and we were worried about him," Davey said. "We were in contact with the American Embassy telling them we were worried about him. The American authorities said, 'He's over 18 we can't do anything.'"

Sandford left school when he was 15, his father said, "because he couldn't cope with it all, so he's got no qualifications or job experience."

As a teenager, Sandford was a massive fan of the British television series Robot Wars and spent thousands buying robots from the show, according to the Mirror tabloid.

John Findlay, an expert from the show, told the paper that Sandford was active in the robot community and that he believed he suffered from autism. In June 2014, Sandford messaged Findlay on Facebook and told him he had a "financial crisis" and wanted to sell his robots.

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"He was calm, very quiet, a little bit weird and reserved," Findlay said.

Originally from Dorking, a town about 40km south of London, Sandford moved to New Jersey about 15 months ago, his father said. He had met an American woman in England and moved abroad to be with her. Davey said he didn't know if his son was still with the woman.

"Looking back, I don't want to use the term radicalised, but we don't know who he has been speaking with. This just isn't him," his father said.

Davey said his son seemed upset in recent months but that he didn't know why. But he insisted that Sandford didn't have any violent tendencies.

"He's a nice kid and literally wouldn't hurt a fly," Davey said. "He used to tell us not to use fly spray because he didn't want any flies to die."

A spokesman for Britain's Foreign Office said, "We are providing assistance following the arrest of a British national in Las Vegas."